The present invention relates to a tandem-type image forming apparatus comprising image forming stations for respective colors which are arranged along a transfer belt and each of which is composed of an image carrier, and a charging means and a developing means which are arranged around the image carrier. The image forming apparatus forms a color image by passing the transfer belt through the respective stations.
As an example of such tandem-type image forming apparatuses as mentioned above, Japanese Patent Unexamined Publication No. H11-95520 describes an image forming apparatus comprising a transfer belt (paper carrying belt) which is laid around plural rollers with some tension to extend obliquely, plural image forming stations which are arranged in the tensioning direction of the transfer belt, and plural laser scanning means which are arranged to be partially superposed on each other, thus achieving the reduction in size of the entire apparatus. Disclosed in the publication is a technology for utilizing a space obliquely below the tensioned transfer belt as a space for mounting a sheet reversing path for dual-side printing or a sheet cassette or a space for allowing the operation for removing a jammed recording medium if jam occurs.
In addition, Japanese Patent Unexamined Publication No. 2001-249522 describes an image forming apparatus comprising a transfer belt (intermediate transfer belt) which is horizontally laid around plural rollers with some tension, plural image forming stations which are arranged to face a lower tensioned surface of the transfer belt, and writing heads of array type which are arranged below the respective image forming stations. Disclosed in the publication is a technology of shortening a recording medium carrying path by the aforementioned arrangement, whereby the area to be required to open for removing a jammed recording medium if jam occurs can be reduced, thus achieving the reduction in entire size of the image forming apparatus.
The reduction in entire size of the image forming apparatus is achieved by arranging the transfer belt obliquely or by employing the writing heads of array type in the aforementioned conventional technology. However, the arrangement of electrical circuit for operating an image forming apparatus is an important issue for further reducing the size of the apparatus.
Particularly in an tandem-type image forming apparatus just like the conventional apparatus as mentioned above, high-voltage power supply substrates for charging means, developing means, transfer means, and the like, a power supply substrate for mechanically operating image forming process, and an interface substrate for separating image data from a host computer into data for respective colors and conducting image processing should be large in order to operate the plural image forming stations at once. Therefore, as these substrates are disposed on an upper or lower portion of a side face or a back face of the image forming apparatus body, the miniaturization of apparatus turns out to be impossible.
Further, the aforementioned conventional methods have problems as mentioned below on the maintenance of apparatus. In the method of Japanese Patent Unexamined Publication No. H11-95520, if jam of a recording medium occurs, it is required to detach the image forming stations in the axial direction of image carriers from the apparatus before removing a jammed recording medium. Accordingly, not only it is troublesome to remove the jammed recording medium, but also there is a possibility that surfaces of the image carriers and the transfer belt may be scratched, causing image defects. The transfer belt can be evacuated by pivotal movement before removing a jammed recording medium. In this case, however, a space where the transfer belt is evacuated is necessary. This also avoids the miniaturization of apparatus. Openings for allowing the image forming stations to be inserted or removed in the axial direction of the image carriers and for allowing the removal of a jammed recording medium are required to be formed in a frame for properly positioning and supporting the respective components, thus reducing the rigidity of the frame. This may deteriorate the quality of obtained image.
On the other hand, the method of Japanese Patent Unexamined Publication No. 2001-249522 has a problem that the apparatus having an opening for replacing the image forming stations and an opening for removing a jammed recording medium which are separately provided along different directions and it is difficult for users to know which opening has to be handled by an operator for maintenance. In addition, since the transfer belt must be detached from the image forming apparatus body before replacing the image forming stations, an operator should prepare a space for temporally placing the transfer belt around the image forming apparatus before the replacement of the image forming stations. Therefore, there are problems that it is troublesome to replace the image forming stations and that the transfer belt may be scratched when temporally placed.
To solve the aforementioned problems, the present applicant has proposed, as Japanese Patent Application No. 2002-209615 (also filed in the United States), an image forming apparatus which comprises image forming stations for respective colors arranged along a transfer belt, each image forming station including an image carrier, a charging means and a developing means disposed around the image carrier, and is characterized in that said transfer belt is arranged obliquely and by further comprising a housing body in which the image forming stations and the transfer belt are arranged, and a frame which is attached to the housing body such that the frame is pivotally movable, wherein the transfer belt is attached to the frame. Accordingly, it is possible to provide an image forming apparatus which is quite compact and is quite ease of maintenance.
In the image forming apparatus proposed in the aforementioned application, an image density detecting means is held, which adjusts the positions of toner images for respective colors on the transfer belt, detects the densities of the respective toner images, and compensates color registration errors of the respective images and the image densities. A holding portion for holding the image density detecting means is provided with a mechanism, composed of screws or levers, for adjusting the position of a sensor relative to the intermediate transfer belt.
However, the image forming apparatus proposed in the aforementioned application has such a structure that the transfer belt must be detached from the housing body every time the maintenance is required. Accordingly, there is a problem that the position of the image density detecting means may be shifted when the transfer belt is returned to its original position by pivotal movement, with the result that the distance between the image density detecting means and the transfer belt is changed, thus deteriorating the detecting accuracy.
It is an object of the present invention to provide an image forming apparatus which can solve the aforementioned conventional problems and which is detachable by pivotal movement relative to a housing body and can prevent the deterioration in detecting accuracy of the image density detecting means.